Town gases and coke oven gases, which were obtained by thermal processes, were used for a long time in the past as gaseous fuels, both for public lighting and for domestic requirements. These gases comprised highly odoriferous components. Consequently, they had a strong specific odour, so that a gas leak could be easily detected.
In contrast, the gaseous fuels currently used, whether natural gas, propane, butane, liquefied petroleum gas (or LPG) or even oxygen (for example for welding operations), are essentially odourless, either because of their origin or because of the purification treatment which they have received.
Thus, if leaks are not noticed in time, the rapid formation occurs of mixtures of gaseous fuels and of air which can explode, with consequently a high risk potential.
It is for these safety reasons that the natural gas moving in gas pipelines is odorized by injection (in specialized stations) of appropriate additives known as odorants.
Natural gas is generally conveyed odourless, after an appropriate purification treatment, from the production sites to the consuming countries, either via gas pipelines or (in liquid state) in specialized ships (methane carriers). In France, for example, natural gas is thus received in a limited number of injection stations where the odorant is injected, so that the natural gas, both that which moves through the French gas pipeline network and that which is stored in underground tanks, is odorized, which makes possible easy detection in the event of a leak, wherever in the network it occurs.
In other countries, natural gas may be distributed over the area of the country by a network of pipelines in which it moves without odorant, the natural gas then being odorized as it enters the towns in which it is consumed, which requires an even higher number of injection stations.
Storage tanks are generally maintained under an atmosphere of nitrogen or of natural gas in order to limit, at this stage, the risks of explosion.
Alkyl sulphides, used as odorants, alone or as a mixture, are known. Mention may be made, for example, of diethyl sulphide, dimethyl sulphide, methyl ethyl sulphide or tetrahydrothiophene, which are widely used for their excellent properties, in particular which are capable of triggering a feeling of alarm in people in the event of an accidental leak of natural gas thus odorized and of initiating the necessary protective operations.
However, during the combustion of natural gas, these products generate an amount of sulphur dioxide which, however low it might be, becomes not insignificant when an overall balance is carried out on the scale of a country or of a region, in particular with a high level of industrialization or urbanization. Thus, by way of example, the combustion of a natural gas odorized with tetrahydrothiophene at a concentration of 10 mg/Sm3 (or number of m3 of the gas, measured under standard temperature and pressure conditions) generates 7.3 mg/Sm3 of sulphur dioxide.
In the general context of better consideration of environmental constraints, it is therefore desirable to reduce the amounts of SO2 discharged to the ecosphere via odorants based on alkyl sulphides present in natural gas during the combustion of the latter.
Furthermore, the use of alkyl acrylates as components of gas odorant mixtures is disclosed in the literature.
Thus, Patent Application DE 19837066 mentions a process for the odorization of natural gas by addition of a mixture comprising an alkyl acrylate, a nitrogenous compound of pyrazine type and an antioxidant. However, this mixture exhibits the disadvantage of not having an odour characteristic of gas and is thus capable of being misinterpreted in the event of a gas leak. The risk is, of course, the non-detection of this leak and an explosion, if the concentration of gas in the air reaches its lower explosive limit.
Patent JP55-137190 also discloses an odorant mixture combining ethyl acrylate with a specific sulphur compound, in this instance tert-butyl mercaptan (or TBM). However, the major disadvantage of this mixture is that, because of the chemical reactivity of TBM with ethyl acrylate, the 2 components of the odorant mixture have to be stored in the various injection stations in separate tanks and also require separate injection pumps and heads for introduction into the gas pipeline. This results, in view of the complex logistics for the odorization of natural gas set out above, in a considerable increase in the costs of the injection stations resulting from the necessary multiplication in the storage tanks and injection pumps and heads.
Furthermore, Patent Application WO 2004/024852 discloses an odorant consisting of four components, including an alkyl acrylate, an alkyl sulphide and an antioxidizing stabilizing agent, such as tert-butylhydroxytoluene, hydroquinone, and the like.
It is an aim of the present invention to provide a novel odorant mixture which overcomes in particular the disadvantages of the odorant mixtures of the prior art set out above.